1. Technical Field
The present invention relates to a vibration gyro sensor for detecting angular velocities.
2. Related Art
Car navigation devices specifies the position of a vehicle based on the angular velocity information provided by gyro sensors, in order to detect the direction of motion in autonomous navigation. Generally, these gyro sensors output analog signals which correspond to the magnitude of angular velocities. The analog signals are converted into digital signals with an A/D converter, and the digital signals are retrieved into a navigation system, so as to be used in arithmetic processing. During the conversion of analog signals to digital signals, the fluctuation of a power voltage of the A/D converter may cause a fluctuation in a reference voltage of the A/D converter and in a voltage width of each resolution of the A/D converter. Thus, gyro sensors provided with ratiometric functions are suggested, in order to vary the analog signal being output from the gyro sensor, in accordance with the fluctuation of a power voltage, making a detection signal dependent on the power voltage.
For instance, a gyro sensor which has a multiplier circuit for amplifying a detection signal proportionally to a power voltage has embodied a ratiometric function with a drive signal dependent on a power voltage. Instead, with the detection signal dependent on the power voltage, the decline in dependency accuracy with respect to the power voltage is prevented, the decline being caused by a conversion error in an electrical/mechanical and mechanical/electrical conversion (refer to JP-A-2006-10408 and JP-A-2001-296140 for examples).
However, detection signals and power voltages contain noises that include many frequency components. The multiplier circuit referred to in JP-A-2006-10408 amplifies a detection signal input from a wiring line, in proportion to a divided voltage Ve which is dependent on a power voltage. It is known that, in multiplier circuits, multiplying a signal with another signal, both having the same frequency component, produces a signal having doubled frequency component and a direct current. In other words, if two input signals are both sin A, then sin A*sin A becomes (1−cos 2A/2). Therefore, both the detection signal and the power noise receive the same frequency component. As a result, the error component of the direct current is added to the detection signal, thereby lowering the measurement accuracy.
Moreover, JP-A-2001-296140 discloses a structure in which a filter circuit is installed before an absolute voltage-ratio conversion circuit. However, this absolute voltage-ratio conversion circuit is driven by two differential voltages. One differential voltage represents a difference between a reference voltage input from an absolute reference voltage circuit and an input signal input from a filter. Another differential voltage represents a difference between an output voltage of a voltage divider circuit and an output voltage of a sub voltage divider circuit. In other words, circuits such as differential amplifier circuit that outputs differential voltage is needed before a circuit that realizes the ratiometric function. Consequently, in the event of power noises being generated in a circuit provided subsequently to the filter, the measurement accuracy declines, since an error component of a direct current is added to a detection signal. This decline is caused by the power noises that have the same frequency components being input into a power source as well as into an input signal of a circuit that realizes the ratiometric function.